1. Field of the Invention
The invention relates to a process and a device for drying solid materials, such as, for example, brown coal, peat, sand, filter cakes from mechanical separation processes and sludges which contain less than 98% % by weight [sic] of an evaporable material, for example water, in which an indirectly heated fluidized bed is formed and contains solid material fluidized by a fluidizing medium, the fluidizing medium being the evaporable material in vapor form, and in which the dried material discharged from the fluidized-bed drier can be fed, if required after cooling, for further processing, utilization or disposal in a landfill, but the evaporated material can be fed to a purification, cooling, physical utilization and/or heat recovery stage and is suitable for use in industry, the building industry, agriculture and communal disposal.
2. Description of Related Art
Drying processes, in particular those which separate off water as an evaporable component from solid materials, are of considerable economic and social importance for industrial production, the building industry, energy conversion and disposal in communities and plants. In some cases, the drying is so much a matter of course and so integrated in the process sequences, as in the combustion of water-containing fuels, for example brown coal, and of sludges, that the environmental pollution caused by it through increased energy demand and increased emission are regarded as quite natural.
For the utilization of crude brown coal for energy, milling/drying plants are employed, particularly in power stations, said plants sucking back a part of the furnace gas, produced in the furnace, as a heat energy medium for the milling/drying, so that the water in the coal evaporates as a result of heat transfer by furnace gas at 800.degree. to 1000.degree. C. with crude brown coal in the stack gas stream, before or during milling of the coal to form pulverized fuel. The relevant prior art is described in detail in the book by Effenberger, H. "Dampferzeuger" [Steam generators], VEB Verlag fur Grundstoffindustrie, 1st Edition, 1987. Based on the heat energy liberated in the furnace of the boiler, this method of drying causes more than 1.5 times the minimum stack gas emission required on the basis of the laws of nature, owing to the high intrinsic fuel requirement of the drying and the proportion of steam in the stack gas.
In the brown coal enrichment, disk and tubular rotary driers indirectly heated with steam, i.e. contact driers, are predominantly used, as described in detail by Krug and Nauendorf in the book "Braunkohlenbrikettierung" [Briquetting of brown coal] Volume 1, section on Drying, VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1984, 1st Edition.
By using steam from tapped turbines or back-pressure steam as a heat medium for the drying, which, through condensation, transfers its latent heat energy indirectly to the coal after it has been converted into saturated steam, for example by injection of condensate, the known principle of "Power/Heat coupling" is utilized and a reduction in the fuel requirement to be attributed to the drying is achieved. Compared with the milling/drying conventionally used in the brown coal power stations, it was therefore possible to reduce the comparable sum of the stack gas emission occurring during the individual enrichment and utilization sections to approximately 1.3 times the minimum required on the basis of the laws of nature. However, since "entrained air" is used in most cases, these advantages are not effective.
The introduction of indirectly heated disk and tubular rotary driers and hence the development of power/heat coupling, for example in brown coal and peat power stations, have been unsuccessful to date since the required fuel mass flows and the limited capacity of such driers are in contradiction with one another and have not permitted the object to be achieved in an economical manner.
East German Patent 67,770 discloses a process and a means for the predrying of water-containing solid fuels, in particular of soft brown coal, in which the drying of brown coal before its combustion in a steam boiler is carried out in a fluidized-bed drier directly heated with steam. As in the case of the disk and tubular rotary driers, steam from tapped turbines or back-pressure steam should be used here and hence the principle of power/heat coupling utilized.
In East German Patent 67,770, it is assumed that every suitable fluidizing medium, i.e. including steam, can be used for fluidizing the brown coal above the fluidization base in the fluidized-bed drier.
U.S. Pat. No. 3,800,427 describes an indirectly heated fluidized-bed drying process in which the brown coal is fluidized with steam so that the drying takes place in a steam atmosphere. However, the invention assumes that the brown coal is heated in the steam atmosphere to such an extent that the sulfur compounds are eliminated and undergo addition with additives which may be simultaneously present in the fluidized bed.
German Patent 2,901,723 extends the use of a fluidized bed indirectly heated with steam and fluidized with steam generally to include the drying of solid materials which contain less than 95% by weight of an evaporable material. In addition to water, the evaporable material may also be other materials, such as solvents, which in their vapor form are a fluidizing medium and in their saturated vapor form, with utilization of different partial pressures, are also heat media for the indirect heating of the fluidized bed.
Compared with U.S. Pat. No. 3,800,427, German Patent 2,901,723 restricts the permitted temperature of the fluidized bed and specifies that it is essentially below the decomposition temperature of the solid material, so that the vapor removed from the fluidized-bed drier should consist of the evaporable material, essentially without contamination by other gaseous substances.
Research and development work over several years has shown that the process disclosed in German Patent 2,901,723 is not technically realizable in the form described. It has been found in particular that the temperature of the fluidized bed cannot be freely chosen, and that the gaseous impurities of the evaporable material are contained completely in the vapor of the evaporable material emerging from the fluidized-bed drier, practically independently of the fluidized-bed temperature.